A telephone call between the second telephone terminal and the third telephone terminal is sometimes forwarded to the first telephone terminal. For example, when the first telephone terminal and the second telephone terminal belong to the same internal line communication network, the first telephone terminal and the second telephone terminal are managed by the same telephone relay device. It facilitates forwarding from the second telephone terminal to the first telephone terminal.
In some cases, the telephone call is forwarded from the second telephone terminal to the first telephone terminal, even when the first telephone terminal and the second telephone terminal are managed by different telephone relay devices. For example, Patent Documents 1 and 2 describe that a signal line separate from a telephone call channel is provided between the telephone relay device for managing the first telephone terminal and the telephone relay device for managing the second telephone terminal. They also disclose that information (for example, access point information) necessary for forwarding is forwarded through the signal line.